If the column type is enum and you want to convert it to numeric, you should first convert it to character then convert it to numeric. Otherwise, the values may be converted to underlying factor values, not the expected mapped values.
See also
numeric for the base R implementation, as.numeric().
Examples
if (FALSE) { # \dontrun{
library(h2o)
h2o.init()
f <- "https://s3.amazonaws.com/h2o-public-test-data/smalldata/junit/cars_20mpg.csv"
cars <- h2o.importFile(f)
h2o.ascharacter(cars)
h2o.asnumeric(cars)
} # }